The present invention relates to a system for stripping coatings from workpieces which has an integrated water management and acid recycling system and a process for using same.
Gas turbine engines in aircraft are taken out of service at periodic intervals and regular maintenance service is performed on them. Part of the regular repair sequence for the blades and vanes (individual or together referred to hereafter as xe2x80x9cairfoilsxe2x80x9d) of these engines includes the removal and then replacement of the worn coatings from their surfaces. These coatings are usually either an aluminide coating or an MCrAlY coating. The underlying base metal of airfoils are generally made of either a nickel base alloy or a cobalt base alloy. These coatings provide airfoils with a barrier to the hot corrosive environment in which airfoils operate.
In the past, these aluminide and MCrAlY coatings were removed from airfoils by soaking the parts either in nitric acid solutions (to remove aluminide-type coatings) or in hydrochloric acid solutions (to remove MCrAlY-type coatings) in high acid concentrations for up to six hours at elevated temperatures. This soaking process has several disadvantages associated with it.
This soaking process is extremely labor intensive and can produce non-uniform and unpredictable results. It can also damage or destroy airfoils if improperly carried out. Furthermore, each airfoil part requires extensive masking to protect areas sensitive to the acid soaking solution. Such areas include internal surfaces and the root section of the airfoil. These masking operations are costly, add significant time to the repair process and, if not properly carried out, can lead to damaged or destroyed parts. Still further, these soaking processes may result in extensive amounts of acidic waste solution that must be properly disposed of as well as have a long cycle time and require relative large amounts of energy to heat the acidic solutions.
A better airfoil stripping process is needed by the engine maintenance and repair industry. This better airfoil stripping process should be one that has a reduced cycle time; requires reduced amount of labor; requires less masking and lower operating temperatures; produces less hazardous waste effluent; requires less heating energy; produces uniform and predictable stripping results so that fewer parts are damaged, destroyed or require recycling. Such a stripping process has been presented in allowed co-pending U.S. patent application Ser. No. 09/216,469, filed Dec. 18, 1998, entitled FEEDBACK CONTROLLED STRIPPING OF AIRFOILS. In this process, a coating is electrochemically stripped from an airfoil by immersing the airfoil in an electrochemical acid bath for a sufficient period of time to remove the coating from the airfoil while maintaining a controlled absolute electrical potential with respect to a reference electrode on the airfoil surface.
In order to make the stripping process commercial, the expended stripping solution and the waste water that is created from rinsing the stripping solution must be managed. In the past this has required the use of large industrial waste water treatment plants.
Accordingly, it is an object of the present invention to provide a feedback controlled stripping system with an integrated water management and electrolyte recycling system which can be used to strip or remove coatings from a wide variety of workpieces.
It is a further object of the present invention to provide a process for utilizing the system to strip coatings from a wide variety of workpieces.
The foregoing objects are attained by the system and the process of the present invention.
In accordance with the present invention, a feedback controlled stripping system is provided with integrated electrolyte recycling. This allows protective coatings to be removed from turbine blades, vanes, and other workpieces, as well as permitting brazing and solder compounds to be removed from metals, in cold, dilute acid without masking by the use of controlled potential stripping. The integration of a recycling system based on acid distillation stabilizes the chemistry of the stripping solution while minimizing the volume of chemical waste created by the process. The integration of zero waste water discharge equipment allows the system to be located in facilities lacking central waste water treatment plants.
The coating removal system of the present invention broadly comprises a stripping tank containing an electrolyte bath stripping solution for removing a coating from at least one workpiece immersed in the electrolyte bath while a controlled absolute electrical potential is maintained on the at least one workpiece with respect to a reference electrode immersed in the electrolyte bath; a rinse tank containing a rinse solution for rinsing the at least one workpiece after completion of removal of the coating from the at least one workpiece; and a distillation unit for receiving used electrolyte from the stripping tank containing dissolved metals, for purifying the electrolyte received from the stripping tank and for returning the electrolyte in a purified form to the stripping tank. In a commercial embodiment, the stripping tank, rinse tank, and distillation unit are mounted on a skid. The coating removal system further comprises a control module for operating the system.
A process for removing a coating from a workpiece using an acid bath stripping solution and for regenerating the stripping solution broadly comprises the steps of stripping a coating from a workpiece by immersing the workpiece in an electrochemical acid bath for a period of time sufficient to remove the coating from the workpiece while the workpiece in the electrochemical bath is maintained with a controlled absolute electrical potential with respect to a reference electrode and regenerating the electrochemical acid bath by atmospheric distillation of the electrochemical acid bath.
Other details of the system and the process of the present invention, as well as other objects and advantages attendant thereto, are set forth in the following detailed description and the accompanying drawings in which like reference numerals depict like elements.